Host image used in email template in app

Right now the crown logo in email templates is hosted on Github. Github
is not a CDN.

For now, hosting it in the app is a better solution. At some point we
should have a CDN for all assets on the app, which would be even better.

https://www.pivotaltracker.com/story/show/116952911

tests/app/test_assets.py

@@ -0,0 +1,9 @@
+import app
+
+
+def test_crown_logo(app_):
+    with app_.test_request_context():
+        #  This image is used by the email templates, so we should be really careful to make
+        #  sure that its always there.
+        response = app_.test_client().get('/static/images/email-template/crown-32px.gif')
+    assert response.status_code == 200